A material obtained by forming a thick silicon dioxide film on a silicon substrate has been used for the devices that are used in such fields as soft photonics which is chiefly concerned to optical data communication and processing technology, amenity photonics which aims at utilizing a light for the optical display and optical input/output which are easy for human beings and environment to use, and hard photonics challenging ultimateness of a new source of light and developing a technology for utilizing optical measurement.
An optical waveguide in an optical integrated device, for example, is formed by burying a core layer having an optical waveguide circuit pattern in a silicon dioxide film on a silicon substrate. In this case, the silicon dioxide film must have a thickness which is at least not smaller than ten and several microns if the thickness of the core layer is selected to be, for example, the almost same size as that of an optical fiber that is connected.
A typical method of forming the silicon dioxide film on the silicon substrate can be represented by a well-known direct thermal oxidation method. The direct thermal oxidation method is to form a silicon dioxide film by directly thermally oxidizing the surface of the silicon substrate. The thickness of the film that is formed varies in proportion to the time of oxidation when it has a small thickness. However, since the oxidation reaction is conducted through the oxide film that has been formed, formation of the film having a large thickness takes a time in proportion to the one-half power of the time of oxidation. It is therefore difficult to form a film having a thickness of as large as ten and several microns or more. In order to increase the rate of oxidation, therefore, there has been put into practical use a high-pressure oxidation method in an oxidizing atmosphere of ten and several atms to 25 atms. However, this method is accompanied by such barriers as legal restrictions related to high pressures and expensive facility costs. It is therefore virtually difficult to form a silicon dioxide layer having a thickness of as large as ten and several microns to 100 microns or more.
In order to solve the above problems, the present applicant has developed a method of forming a silicon dioxide film by the deposition of polysilicon (Japanese Patent Application No. 342893/2000 “Method of Forming a Silicon Dioxide Film and Method of Forming Optical Waveguide”). This method is to form a silicon dioxide layer of a desired thickness by depositing polysilicon on a silicon substrate, followed by thermal oxidation treatment to form a silicon dioxide film, then, newly depositing polysilicon on the thus formed silicon dioxide film, followed by thermal oxidation treatment to form a silicon dioxide film, and repeating the above operations.